A charged particle beam device is available as a technique of observing a circuit pattern formed on a sample such as a semiconductor wafer. The charged particle beam device irradiates a sample with a primary charged particle beam and detects secondary charged particles generated thereby. The detected secondary charged particles are converted into an image which is shown on a display unit. If the primary charged particle beam is an electron beam, the device is a scanning electron microscope (hereinafter abbreviated as SEM). First, electrons emitted from a heating type or field emission type electron source are accelerated. Then, an electron beam whose diameter is decreased by a lens is formed. By scanning a sample (for example, a wafer, reticle, etc) two-dimensionally with the electron beam and detecting the generated secondary electrons, a two-dimensional scanned electron image of a fine pattern on the sample is obtained.
At this time, depending on the landing energy of primary electrons onto the sample, the penetration of electrons into the sample will spread deep, resulting in deterioration of the resolution of the image obtained by detection of secondary electrons. Therefore, the landing energy must be decreased. Furthermore, among the materials used in the process, the number of materials susceptible to an electron beam is increasing. For this reason, it is indispensable to decrease the energy of incident electrons. In addition, since the efficiency of generation of secondary electrons varies depending on the landing energy of incident electrons onto the sample, charge-up of the sample occurs. Therefore, the landing energy must be selected appropriately to keep the efficiency of generation of secondary electrons constant. In order to resolve all the problems related to resolution, susceptibility to an electron beam, and charge-up, it is essential to decrease the energy.
The use of a length measuring SEM (Critical-Dimension Scanning Electron Microscope, hereinafter abbreviated as CD-SEM) is the mainstream of dimensional measurement of semiconductor device patterns. With the recent tendency toward the miniaturization of circuit patterns, high resolution is demanded. A mechanism in which after generation of electrons from the electron source, the electrons are accelerated and before the electrons enter the sample, a decelerating electric field is applied (application of a retarding voltage) is provided. This makes it possible to achieve both high resolution of an obtained image and low acceleration of landing energy.
However, recently among samples to be measured, electrostatically charged samples or samples which become electrostatically charged by beam irradiation have been emerging and due to the electrostatic charge of the sample or its distribution, a focusing error or astigmatism occurs in the CD-SEM. This kind of electrostatic charge may distort the trajectory of the primary charged particle beam to irradiate the sample or if the primary charged particle beam is converged by an electromagnetic lens, it may displace the focal point on the sample surface. This requires time to adjust the electromagnetic lens and the like for focus position and astigmatic readjustments. Furthermore, if the abovementioned electrostatic charge has an in-plane distribution on the sample, focus position and astigmatic readjustments are required each time the point of irradiation with the primary charged particle beam moves to each measuring point on the sample. As a consequence, throughput is remarkably decreased in the measurement and inspection of fine patterns with the charged particle beam of a CD-SEM or the like.
As a solution to this problem, Patent Document 1 discloses a technique which measures, by image processing, the astigmatic difference (range finding between astigmatic focal lengths) based on the distance between two points at which the contrast of differential images in perpendicular directions at different focus positions is the maximum. Patent Document 2 discloses a technique which finds a point at which the amount of movement for defocusing is the minimum, by making an astigmatism correction in a perpendicular direction with respect to the axial beam.